1. Field of the Invention
The invention relates to a method of rapidly dechucking a semiconductor wafer from an electrostatic chuck. More particularly, the invention relates to a method of dechucking a wafer from an electrostatic chuck by using a hysteretic discharging cycle.
2. Description of the Background Art
Electrostatic chucks (ESC) are used for holding a workpiece in various applications ranging from holding a sheet of paper in a computer graphics plotter to holding a semiconductor wafer within a semiconductor fabrication process chamber. Although electrostatic chucks vary in design, they all are based on the principle of applying a voltage to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrode(s), respectively. The electrostatic attractive force between the opposite charges presses the workpiece against the chuck, thereby retaining the workpiece.
One problem that arises with the use of an ESC is the difficulty of removing the residual electrostatic force between the workpiece and the chuck during "dechucking". This residual force results from electric charges having accumulated at the interface between the workpiece and the ESC support surface. One technique for dechucking involves connecting both the electrode and the workpiece to ground. Another technique reverses the polarity of the DC chucking voltage applied to the electrodes to discharge the electrodes. However, these techniques are not completely effective at removing all the charge on the electrodes and the wafer. Consequently, a mechanical force is often needed to overcome the remaining attractive electrostatic force due to residual charges on the electrodes and wafer. At times, the mechanical force used to release the wafer may cause the wafer to "pop", i.e., to be released from the chuck in some unpredictable manner, which may cause either wafer damage or difficulty in retrieving the wafer from unintended position. Therefore, a successful dechucking operation is one which leaves the wafer in a state subject to acceptably low residual electrostatic attractive force without "popping" the wafer.
Furthermore, when using these common dechucking practices that simply remove the potential difference between the electrodes, the time required to dechuck by these methods is defined by the RC time constant of the high voltage electrical circuit as well as the effective charge relaxation time constant of the electrode-wafer system. This time constant is given by the effective resistivity-permitivity product, .tau..sub.cr =.rho..epsilon..sub.o.epsilon..sub.r, where .rho. is the volume resistivity of the dielectric layer between the electrode and the wafer (i.e., the chuck material), .epsilon..sub.o is the permittivity of free space, and .epsilon..sub.r is the relative permittivity of the dielectric layer. The total time required to dechuck by this method is often on the order of several seconds.
Other dechucking methods include applying to the ESC an optimal, but non-zero, dechucking voltage. A commonly-assigned U.S. Pat. No. 5,459,632 issued to Birang et al. on Oct. 17, 1995, and is herein incorporated by reference, discloses a method of determining and applying an optimal dechucking voltage which has the same polarity as that of the chucking voltage. Another method involves determining and applying an optimal dechucking voltage for an optimal dechucking period. This is the subject of another commonly-assigned U.S. Pat. application Ser. No. 08/696,293, filed by Loo on Aug. 13, 1998, and is herein incorporated by reference. Although these prior art dechucking methods are effective at removing the residual charge from the wafer and electrodes, none of these prior art references are concerned with very rapid dechucking, i.e., on the order of a hundred milliseconds.
The need for a rapid dechucking operation is most keenly felt in single wafer processing systems, where the advantage of excellent wafer to wafer uniformity is achieved at the expense of a reduced wafer throughput. In a system such as a single wafer ion implant system, the time required to dechuck a wafer can have a substantial impact on the throughput of the system. In U.S. Pat. No. 5,444,597 issued to Blake et al. On Aug. 22, 1995, a bipolar electrostatic chuck is described that retains a wafer in an ion implant system. The bipolar chuck has a pair of electrodes embedded in the dielectric chuck material and applies a differential voltage to the electrode pair to achieve charge accumulation on the backside of the wafer and, ultimately, an electrostatic force to retain the wafer. The wafer is dechucked by applying a DC voltage having a polarity opposite that of the chucking voltage, then applying a switched DC waveform (in effect, an AC waveform) to remove any residual charge. Because the dechucking voltage is directly connected to the electrode pair and not to the wafer, the wafer forms, in essence, a floating electrode. As such, charge is removed indirectly from the wafer. This removal of charge from the wafer is very difficult and can require a substantial amount of time as the interface discharges.
Additionally, the dechucking duration is exacerbated in systems where the platen supporting the wafer is vertically oriented during wafer processing, e.g., ion implant equipment. In these systems, gravity does not aid the chucking process and, to ensure sufficient wafer retention while moving the platen to a vertical orientation, a very strong chucking force is required. Such a strong chucking force requires a substantial chucking voltage to be applied to the electrostatic chuck, i.e., typically much larger than the chucking voltage used in horizontally oriented chucks. This high chucking voltage causes a substantial residual charge to reside between the wafer and the chuck after the chucking voltage is deactivated. The use of conventional dechucking techniques does not remove this large residual charge in a short period of time. As such, the throughput of the wafer processing system is detrimentally impacted.
Therefore, a need exists in the art for alternative methods for chucking a wafer and rapidly dechucking the wafer from an electrostatic chuck used in a single wafer system.